Papers

AVS2004 Session MI-TuM: Spintronics

Tuesday, November 16, 2004 8:20 AM in Room 304A

Tuesday Morning

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8:20 AM		MI-TuM-1 Semiconductor Spintronics: From Basic Physics towards Spin Devices M. Oestreich, D. Hägele, J. Rudolph, S. Döhrmann, R. Winkler (Universität Hannover, Germany); H.M. Gibbs, G. Khitrova (University of Arizona); D. Schuh, M. Bichler (Technische Universität München, Germany); W. Stolz (Universität Marburg, Germany) The electron spin in semiconductors has become a focus of intense research in the context of spintronic devices. A prime condition for the development of potential applications is the understanding of the spin decoherence, i.e. the loss of spin memory. In the first part of this talk we present the spin dynamics in (110) GaAs quantum wells at high temperatures and put forward a new spin dephasing mechanism that ultimately limits the high temperature spin dephasing times in GaAs quantum wells.¹ In the second part of the talk we demonstrate the reduction of the threshold of semiconductor lasers by injection of spin polarized electrons, compare high and low temperature operation, and discuss problems and prospects of these spintronic devices.². ¹ S. Döhrmann et al., "Anomalous spin dephasing in (110) GaAs quantum wells: anisotropy and intersubband effects", cond-mat 0403052. ² J. Rudolph et al., "Laser threshold reduction in a spintronic device", Appl. Phys. Lett. 82, 4516 (2003).
9:00 AM		MI-TuM-3 Characterization of Thin Film MnGa/GaAs(001) Heterostructures J.L. Hilton, B.D. Schultz, S. McKernan, C.J. Palmstrøm (University of Minnesota) MnGa thin films are desirable for use as ferromagnetic contacts in spintronic devices because they can be grown epitaxially on GaAs with perpendicular magnetization¹. The interface between the ferromagnetic contact and the semiconductor has a significant influence on the spin injection efficiency of spintronic devices. It has been shown previously that elemental Mn is not stable on GaAs and that it reacts to form an interfacial region composed of Mn₂As and MnGa², suggesting that MnGa may be stable in contact with GaAs. However, bulk material studies suggest that Mn₂As and elemental Ga are the two stable phases in contact with GaAs³. To address this discrepancy, a number of MnGa/GaAs heterostructures were grown by MBE and subsequently annealed either in-situ or ex-situ for different times and temperatures. X-ray diffraction of MnGa/GaAs samples following growth shows peaks corresponding to both MnGa(001) planes and the GaAs substrate. Following post-growth anneals at 400°C for 1 hr, peaks corresponding to (001) planes of the Mn₂As-like crystal structure are observed. Rutherford backscattering spectrometry shows only minor compositional changes upon annealing, indicating that any reactions are confined to the interfacial region. These results will be combined with results from in-situ RHEED, LEED, STM, and XPS, and ex-situ RBS channeling and TEM to characterize the growth and interfacial properties of epitaxial MnGa/GaAs heterostructures. Supported by ONR, DARPA, NSF, and AFOSR. ¹M. Tanaka et al., Appl. Phys. Lett. 62, 1565 (1993). ²J. L. Hilton et al., Appl. Phys. Lett. 84, 3145 (2004). ³P. Kordos et al., Solid State Electron 18, 223 (1975).
9:20 AM		MI-TuM-4 Co₂MnGe/Ga_1-xAl_xAs Heterostructures: Growth, Characterization and Spin Injection X.Y. Dong, C. Adelmann, J. Strand, X. Lou, S. McKernan, J.Q. Xie, B.D. Schultz (University of Minnesota); A.K. Petford-Long (University of Oxford); P.A. Crowell, C.J. Palmström (University of Minnesota) A number of ferromagnetic Heusler alloys of the type MMnX ("half" Heusler) and M₂MnX ("full" Heusler) have been predicted to be half-metallic ¹. The ability to grow Co₂MnGe epitaxially on GaAs, the predicted half-metallicity and the high Curie temperature, make it an ideal candidate for a spin injecting contact. Co₂MnGe epitaxial films were grown by molecular beam epitaxy (MBE) on Ga_1-xAl_xAs (001) surfaces prepared in a separate MBE-growth chamber and transferred in ultra high vacuum (<10^-10 torr) to the Heusler alloy growth MBE chamber. In-situ RHEED, ex-situ XRD and TEM demonstrate the epitaxial single crystallinity of the films. In-plane VSM measurements showed that the Co₂MnGe films have a 1000 emu/cm³ saturation magnetization at room temperature and a 8 Oe coercivity. A SQUID magnetometer was used to measure the out of plane magnetization, which was found to saturate around 1 Tesla. In order to measure the spin injection, tunneling Schottky barrier contact spin-LED structures were fabricated from MBE-grown p-Ga_0.9Al_0.1As/GaAs(100Å)/n-Ga_0.9Al_0.1As/Co₂MnGe/Al heterostructures. The 70Å thick Co₂MnGe Schottky barrier injector was grown at 175°C and the 25Å thick Al capping layer used to prevent oxidation during exposure to air was grown at 0°C. The epitaxial heterostructures were processed into LED devices and the devices were operated with the Schottky contact under reverse bias and the p-i-n LED under forward bias. Electroluminescence was collected along the sample normal. The circular polarization of the observed electroluminescence was 14% indicating a spin injection efficiency of 14% at 2K. To our knowledge, this is the first time demonstration of spin-injection from a Heusler alloy into a semiconductor. ¹ S. Fujii, S. Sugimura, S. Ishida, and S. Asano., J. Phys.:Condens. Matter 2, 8583 (1990).
9:40 AM		MI-TuM-5 Determination of the Influence of the Interfacial Formation and the Semiconductor Doping Profiles on the Spin Injection from Fe_xCo_1-x Contacts into Ga_xAl_1-xAs C. Adelmann, X.Y. Dong, B.D. Schultz, C.J. Palmstrøm, J. Strand, X. Lou, P.A. Crowell (University of Minnesota); S. Park, M.R. Fitzsimmons (Los Alamos National Laboratory) Spin injection from ferromagnetic contacts into semiconductor structures is a crucial part in spintronic devices operating at room temperature. Recently, it has been shown that spin injection is possible from Fe into GaAs by tunneling through a reverse-biased Schottky contact into an light emitting diode¹. The dependence of spin injection on the inter-face doping level and drift layer doping was studied. Efficient spin injection was only obtained in a narrow interface doping window between 3E18 and 5E18 cm^-3. The optimum drift layer doping was found to be about 1E16 cm^-3. The spin detection efficiency was also found to depend on the p-layer. The carrier transport as a function of doping level will be discussed. The effect of growth temperature and annealing on the spin injection was also investigated. Low temperature annealing was found to increase the electroluminescence polarization. However, at high annealing temperatures, no spin injection was observed suggesting reactions between GaAs and the metal contact. The observed changes in electroluminescence polarization were found to correlate with the changes in the interfacial magnetic properties for Fe_0.5Co_0.5/GaAs heterostructures determined from polarized neutron reflectivity. Optimized devices were found to lead to >10% spin injection at room temperature. This work was supported by the DARPA SPINS program, ONR, and the University of Minnesota NSF-MRSEC program. ¹ A.T. Hanbicki et al., Appl. Phys. Lett. 80, 1240 (2002).
10:00 AM		MI-TuM-6 Growth and Magnetic Properties of Group-IV Dilute Magnetic Semiconductors Y.F. Chiang, R.K. Kawakami (University of California, Riverside) The synthesis of magnetically-doped semiconductors is important for electronics based on spin. We utilize molecular beam epitaxy (MBE) to incorporate magnetic dopants such as Mn and Co into Ge and Si semiconductor thin films. The structural properties of the samples are characterized by in situ reflection high energy electron diffraction (RHEED), x-ray diffraction, and transmission electron microscopy (TEM). Substrate temperatures during growth are monitored by a transferable thermocouple to ensure accurate thermometry for low-temperature growth (0-250Â° C). Magnetic hysteresis loops are measured by superconducting quantum interference device magnetometry (SQUID) and magneto-optic Kerr effect (MOKE) over a large temperature range (5K â?" 300K) in order to determine the Curie temperature, magnetic anisotropy, and remanence. The dependence of magnetic properties on the magnetic dopant concentration and the growth temperature will be discussed.
10:20 AM		MI-TuM-7 Universal Scaling of Magnetoconductance in Magnetic Nanoconstrictions^* S.-H. Chung (University of Maryland, College Park, Argonne National Laboratory) Large magnetoresistance in ferromagnetic transition metals, half-metallic oxides and magnetic semiconductors connected by nanoconstrictions has recently been observed by several research groups. In this work, we present new results that magnetoconductance in nanometer size constrictions has a universal scaling behavior [1]. The results were obtained for half-metallic ferromagnets formed by nanoconstrictions of CrO₂-CrO₂ and CrO₂-Ni. Analysis of the magnetoconductance versus scaled conductance data for all materials known to exhibits so-called ballistic magnetoresistance suggests that the magnetoconductance of nanoconstrictions follows universal scaling. If the maximum magnetoconductance is normalized to unity and the conductance is scaled to the resistivity of the material, then all data points from the current experiment and others in the literature fall into a universal curve that is independent of the constriction material and the transport mechanism. The results agree with a theory that takes into account the enhancement of spin scattering within a magnetic domain wall in nanoconstriction. The adiabatic spin transport increases as the width of the domain wall increases with the size of nanoconstrictions. This analysis suggests that the large magnetoresistance in the nanoconstrictions of materials even in different conductance regimes may have the same mechanism of spin-ballistic transport through magnetic nanoconstrictions. [1] S.-H. Chung, M. Munoz, N. Garcia, W. F. Egelhoff, and R. D. Gomez, Physical Review Letters vol. 89, 287203 (2002). ^* Supported by the University of Maryland, College Park, NSF and MRSEC, by the Spanish DGICyT, and by the DOE, BES under contract W-31-109-ENG-38. ^** In collaboration with N. Garcia, M. Munoz, W. F. Egelhoff, H. Pandana, and R. D. Gomez .
11:00 AM		MI-TuM-9 Spin-Transfer Torque in a Single Ferromagnetic Layer Y. Ji (Argonne National Lab); T.Y. Chen, C.L. Chien (Johns Hopkins University); M.D. Stiles (National Institute of Standards and Technology) When a spin polarized current passes through a ferromagnet, spin angular momentum can be transferred between the conduction electrons and the magnetization of the ferromagnet. As a result, a torque is imparted on the magnetization, which will be realigned toward the polarization direction of the conduction electrons, an effect called â?ospin-transfer torqueâ?. Previously most theories and experiments explore F/N/F trilayer or F/N multilayer structures, where F denotes a ferromagnet and N denotes a nonmagnetic metal. In low magnetic fields, the trilayers hysteretically switch between parallel and anti-parallel states, as the current is swept between polarities with a current perpendicular to plane (CPP) geometry. In high magnetic fields, reversible dV/dI peaks are observed for only one polarity of the current, and previously interpreted as the onset of spin-wave excitations. The multilayer or trilayer structures have been generally presumed indispensable, since non-collinear magnetizations between a polarizing layer and a receiving layer are required to generate spin torques, and the GMR effect is essential in detecting magnetization reversals. In this work, spin-transfer torque effects in a single ferromagnetic layer are demonstrated, using current injection through a point-contact. Differential resistance peaks are observed in high magnetic fields. The current values corresponding to the peak positions linearly depend on the external field. Hysteretic current-induced switching is observed in low magnetic fields. Systematic variations between low field and high field regions have been investigated and the implications will be discussed. The first author's work was done as a Ph.D. student in Johns Hopkins, supported by NSF DMR00-80031 and DMR97-32763. His current work at Argonne is supported by U.S. DOE BES-MS W-31-109-ENG-38.
11:40 AM		MI-TuM-11 Magneto-Resistance in Epitaxial Nano-Contacts for Spintronic Device Applications D. Pearson, R.A. Lukaszew (University of Toledo) Ballistic magnetoresistance (BMR) research has shown surprising MR effects in electrodeposited Ni nano-contacts at room temperature and low magnetic field¹. A large BMR effect may arise from non-adiabatic spin scattering across very narrow (atomic scale) magnetic domain walls (DW) trapped at nano-contacts². Kent el al³ have studied MR in epitaxial microstructures and found small intrinisc DW related effects only on highly anisotropic films. We have studied nano-contacts in various other epitaxial films. The idea behind our scheme is that epitaxial ferromagnetic thin films may favor non adiabatic spin transport provided that the nano-contact is small enough as predicted by Bruno. We used a similar geometry to that utilized by Chopra and Garcia¹,⁴. The combined shape and magnetocrystalline anisotropies provide the required two states for the magnetization at each side of the constriction. Our results indicate that domain walls do play a role in the magnetoresistance of these nano-bridges. Micromagnetic simulations where carried out on the Ni nanocontacts using OOMMF. We will present our work on epitaxial Ni, FeN and CrO₂ nano-contacts. FeN exhibits enhanced magnetic moment and is an attractive candidate for write-heads. CrO₂ is a half metal with 100 percent polarization and therefore of fundamental interest in these studies. ¹ S. Z. Hua and B. D. Chopra, Phys. Rev. B. 67, 060401(R), 2003. ² P. Bruno, Phys. Rev. Lett. 83, 2425 (1999). ³ Kent, et al., J. Phys: Condens. Matter 13 (2001) R461. ⁴ N. Garcia, M. Munioz, V. V. Osipov, E. V. Ponizovskaya, G. G. Quian, I.G. Saveliev and Y.-W. Zhao, J. Magn. Magn. Mater. 240, 92 (2002).